Apparatus for producing nitric acid



June 25, 1957 F. SCHNUR ETAL APPARATUS FOR PRODUCING NITRIC ACIDOriginal Filed Sept. so, 1949' 4 Sheets-Sheet l Hg. 1

June 1957 F. SCHNUR ETAL 2,797,151

APPARATUS FOR PRODUCING NITRIC ACID Original FiledSept. 30, 1949 4Sheets-Sheet 2 l? 25, v195,7 SCHNUR ET AL 2,797,151

APPARATUS FOR PRODUCING NITRIC ACID Original Filed Sept. 30, 1949 4Sheets-Sheet 3 fig. 4

- I 42 32 T7 Y 28 June 25, 1957 F. SCHNUR ET AL 9 APPARATUS FORPRODUCING NITRIC ACID Original Filed Sepi. so, 1949 4 Sheets-Sheet 4' la Z i lg, H ,6. 4

APPTUS FOR PRODUCING NITRIC ACID Original application September 30,1949, Serial No. 118,893. Divided andthis application July 18, 1952,Serial No. 299,664

Claims priority, application Germany October" 1, 1948 4 Claims. (Cl.23283) This invention relates to the industrial production of nitricacid and more particularly to apparatus particularly adapted for thecarrying out of the steps of oxidation of nitrous compounds and to theabsorption of the nitrogen oxides formed by water and/ or aqueoussolutions of such oxidation products, as described in our copendingapplication for patent Serial No. 118,893, and now abandoned of whichthe present application is a division.

' The invention is of particular interest for the large scalemanufacture of nitric acid from nitrous gases and more especially fromthe gases obtained in the catalytic combustion of ammonia.

It is an object of this invention to improve the manner of oxidizing andabsorbing nitrogen oxides in view of an improvement of the oxidation andabsorption methods hitherto used for this purpose.

As is well known to those skilled in the art, the production of nitricacidfrom nitrous gases such as are obtained for instance in thecatalytic combustion of ammonia, requires an oxidationof these gases andtheir absorption in water or an aqueous solution alreadycontainingnitrogen oxides. In this process the nitric oxide '(NO) has to beconverted into nitrogen dioxide (N02). By absorption of the nitrogendioxide in water, nitric oxide is formed and in this process, accordingto the equation nitric oxide is formed, besides the nitric acid, andthis nitric oxide has to be subjected once more to oxidation.

In the large scale manufacture of nitric acid the oxidation andabsorption of the nitrous gases has hitherto been carried through inacid towers or columns, in which the absorbing liquid, which is water ora dilute acid solution, starting at the top of the tower uniformlytrickles through a plurality of layers of packing material, beingcollected at the bottom ofthe tower in a collecting vessel, from whichit is lifted by a pump, after having been duly cooled, black to the topof the tower. The nitrous gases are introduced into the bottom sectionof the column of packing material and rise in counter-current to theabsorbing liquid to be sucked oil in the top section of the tower. Theliquid in excess in one tower is transferred to the absorbing liquidcirculation in the preceding tower of a row of towers. From the firsttower of the row the liquid in excess is withdrawn in the form ofconcentrated nitric acid which is the desired final product.

The use of wide-roomed oxidation and absorption columns or towers .ofthe kind hitherto in use for the manufacture of nitric acid involves thedrawback, that within these towers the nitrogen oxidation products arepartly re-formed into nitric oxide (NO). If the nitric acid isproduced'from the gases resulting in the combustion of ammonia and thesegases are cooled suificiently and, if necessary, are admixed with air oroxygen, about 25 to 30 percent only of the NOpresent in the gas mixtureentering near the bottom of the tower are oxidized to N02. When thegases arrive at the top of thetower, approximately to percent of themhave been converted into N02. The absorbing liquid circulating in thetower, whose volume is constantly increased by the water of condensationin the gases and by other supplies of liquid, is contacted near the topof the tower with highly oxidized gases, while near the bottom of thetower it meets gases which have only reached a low degree of oxidation.In consequence thereof, the absorption liquid absorbs near the head ofthe tower N02, while retaining NO. Near the bottom of the tower itabsorbs NO and releases N02, which is undesirable. Under thesecircumstances the actual percentage of N02 in the absorption liquid(water or dilute nitric acid) only reaches a value which approximlatelycorresponds to the percentage of NO in the circulating gases asprevailing in the middle between the top and the bottom of the tower.This is the reason why in the oxidation towers hitherto in use theconcentrations of acid obtainable were always lower than should havebeen expected in view of the large space afforded by the tower.

Similar conditions obtained in the absorption of the nitrogen dioxide(N02) formed by oxidation of nitrous gases, i. e. the formation of HNOsitself. Here the ratio of N02 to NO present in the gases is ofimportance. In the high acid towers hitherto in use, in which circulatesthe same body of absorption liquid from the top to the bottom, thedifference in the ratio of N02 to NO between the top part and the bottompart of the tower is so great that the acid withdrawn near the bottom ofthe tower does no by far attain the concentnation of HNO which might beexpected in view of the combustion of the nitrous gases entering thetower, but approximately corresponds only to the ratio of N02 to NOprevailing near the middle or in the top half of the tower.

We have now found, that in the oxidation and absorption of nitrous gasesa more favorable utilization of the inner volume .of the tower and alsoa higher concentration of the nitric acid solutions formed therein canbe obtained, if, in accordance with our invention, the tower issubdivided into a number of superposed, self-contained oxidation andabsorption compartments, each of which is Operated with a liquidcirculation of its own. The height of the individual superposedcirculation compartments within the towerrneeds only-to be so adjustedthat within each compartment the degree ofoxidation as well as therelative ratio of N02 to NO in the nitrous gases rising in thecompartments varies only little. Even ifthe HNOa-concentration of theindividual circulation compartments only corresponds to the conditionsof oxidation in the middle section of each. compartment, the sumof themean values of several consecutive compartmentslin" a towerwill presenta more favorable-oxidation and absorption result, than hithertoobtainable with a liquid circulati on extending inasingle stepthroughout the whole interior of the tower.

If in an individualcase merely an oxidation of the nitrous gases is tobe obtained, it will be particularly advantageous to sochoose' theheight (vertical dimension) of the several tower compartments operatedwith liquid circulations of their own that the compartments and thelayers of packing material in them are lower near the bottom of thetower than those near'the top. In that case it is preferable tosubdivide the inner's'pace of'the tower in such manner that thesuperposed compartments 7 present a greatly increased height, wherebynear the top of the tower, where the oxidation of nitrous gases proceedscomparatively slowly, longer reaction periods than near the bottom'ofthe tower.

of circular, rectangular or polygonal cross-section may be used.Absorption towers of the well-known kind piovided with a centrallyarranged suction pipe for the gases are oifered In the. operation ofourinvention absorption towers" 3 escaping from the tower are adapted to beused in the process. The subdivision of the interior of the tower into anumber of superposed absorption and oxidation compartments requires theprovision of an intermediate bottom for the support of the layers ofpacking material in each compartment. This bottom, apart from supportingthe weight of the packing material, also serves for collecting theabsorption liquid which trickles down, while offering to the gasesrising in this layer sufiicient cross-sectional areas of passage.

The intermediate bottoms can be rendered particularly useful in towersprovided with a central gas suction pipe. In this case the superposedoxidation and'absorption compartments will have an annular cross-sectionand will encircle the central gas suction pipe, while the bottoms aresubdivided into segments, each of which is supported by the outer wallof the tower and by the central pipe. In this arrangement of thecompartments the circulating liquid is preferably fed to the compartmentfrom an annular chamber surrounding the central suction pipe, and thebottom of this chamber is formed with an annular slit, through whichabsorption liquid flows onto a conical distributing body, therebyinsuring uniform distribution of the liquid over the layer of packingmaterial.

We have found that an apparatus constructed substantially as outlinedhereabove is particularly well adapted for the operation of the processof producing nitric acid described and claimed in our copendingapplication identified above which comprises the steps of successivelyacting on a substantially vertically rising current of a gas mixturecontaining nitrogen oxides with a plurality of separate, individuallycooled bodies of circulating oxidizing and absorbing liquid, allowingpart of each body of liquid to descend and mingle with another one ofsaid bodies of liquid and separately collecting the rising gas and thedescending liquors.

In the drawings affixed to this specification and forming part thereof,several embodiments of apparatuses n o I Fig. 4 1s a plan v1ew, drawn toa larger scale, of one of the segments which form the bottom of acompartment,

Fig. 5 being a vertical section on Fig. 4.

Fig. 6 is a vertical section of the parts serving for the distributingof the liquid in a compartment.

Referring to the drawings and first 'to'Fi-g. 1, 1.is the outer wall ofa tower according to our invention, and 2, 3, 4, 5 are four compartmentseach containing a layer of packing material. 6 is the point at which thenitrous gases enter the tower and 7 indicatm the pipe through which thegases escaping from the tower are led into other towers of the series oftowers forming an oxidation and absorption unit.

Underneath the layer 2 of packing material the liquid which has trickleddown through this layer, is exhausted by means of a pump and forced intoa cooler 9 and from the cooler into the distributing device 10 whichdistributes the liquid once more over the packing material layer2.

Underneath the layer of packing material 3 a bell bottom 11 is arrangedon which collectsthe liquid trickling through the layer. Here again apump 12 forces the liquid collecting on the bottom llback through acooler 13 to thedistributing device 14.

The layers-ofpacking'material'4 and 5 are continuthe line VV in ouslysupplied with circulating liquid of their own by means of pumps 15 and17 and coolers 16 and 18, respectively.

From each bell bottom arranged underneath each packing material layerthe liquid collecting on the bottom escapes through an overflow pipe 19to flow through the layer of packing material in the compartment below,ultimately reaching the bottom compartment, from which the highlyconcentrated nitric acid is exhausted by means of the pipe 20.

It is preferable as a rule to operate the liquid circulation in theupper compartments at a lower temperature than in the lower ones. Onemay for instance feed the coolers 18 and 16 with a refrigerant which hasbeen cooled down in a refrigerating device so far that it enters thecompartment with a temperature of 4 C., escaping therefrom with atemperature of about 10 C. The cooling devices 9 and 13 connected withthe compartments nearer the bottom of the tower are supplied with plainwater. Instead of this one may supply to the lower coolers 13 and 9 alsothe refrigerant used in the upper coolers 18 and 16.

The subdivision of the oxidation and absorption towers into severalsuperposed compartments with the liquid circulation and gas absorptionoperated in separate stages olfers the advantage of enabling strongeracid to be obtainable with towers of lesser volume than those hithertoin use. An acid tower according to this invention will enable an acid of'51 percent HNO3 to be obtained, as compared with a tower of equalvolume operated in the manner hitherto known, in which only an acidcontaining 48 percent HNO3 could be produced. It 'is important to notethat it has always been considered particularly diflicult to obtain anacid of a concentration higher than 48 percent HNOa.

Since the individual layers of packing material are operated, accordingto this invention, with separate liquid circulations, there must beprovided underneath each layer an intermediate bottom on which thecirculating liquid can be collected in order to be sucked off by a pumpand forced through a cooler to the distributing device above the layerof packing material. These intermediate bottoms serving to collect theliquid may at the same time support the layer of packing material.

When operating with a central exhaust for the gases, the tower, asillustrated in Fig. 2, has an outer wall 21 and a large, centrallyarranged gas pipe 22 for the exhaust of the gases entering the bottomsection of the tower at 23. Within the tower are arranged three layersof packing material 24, 25 and 26 and underneath each layer is arrangedan intermediate bottom 27 consisting of a number of wedge-shapedsegments 28 (Fig. 3). The inner end of each segment 28 rests on a ringsegment 29 (Fig. 4) mounted on the gas pipe 22, while 'at its outer endit rests on a supporting ledge 30 mounted on vertical supports 31 whichalso serve for reinforcing the outer wall of the tower.

Underneath each bottom segment 28 is arranged a segment trough 32 (Fig.5) extending over the entire length of the bottom and formed with agreat number of lateral openings 33 aflording a passage for the gasesrising in the tower. Each trough is also provided with an overflow pipe34 through which the liquid can flow into an annular collecting pipe 35(Fig. 2) and from there to a pump which forces the liquid through acooler 36 onto a distributing device 37 or 38. From this device theliquid is distributed over the layer of packing material to start afresh circulation through it. The bottom segment 28 is formed withreinforcing edges 39 (Fig. 5). The carrying capacity of the segments 28and of the intermediate bottoms 27 is materially increased by the sidewalls 40 of the troughs 32 below them which are fixedly supported atboth ends by the ledges 23 and 30.

'On the top side of the bottom segments 28 there may be.

provided ledges 41 (Fig. 4) which support rods 42 forming a supportinggrid for the packing material. If desired, a perforated plate (notshown) may be placed on top of these rods.

The radial edges 39 of adjacent bottom segments are covered by strips 43which prevent the liquid from flowing ofl? the segments 28 withoutcollecting in the troughs 32.

Since nitrous gases have a strong corrosive action, packing material ofceramic material is preferable in the oxidation and absorption towers.The considerable weight of this material requires intermediate bottomsof high carrying capacity for the superposed compartments. These bottomsshould therefore be made of acid-proof alloyed steel (for instancechromium-nickel steel). Owing to their segment shape and to thereinforcing edges 39 and 40, the segmental bottoms according to thisinvention have a very high load carrying capacity, while requiringcomparatively little steel material in their construction.

In the individual superposed circulation compartments the absorptionliquid must be distributed uniformly over the layers of packingmaterial. This requirement creates certain difliculties, because thevertical dimensions of the spaces between the several layers arecomparatively low. If the tower is provided with an inner gas suctionpipe, the necessity of distributing the liquid over a large annularcross-sectional area increases these difficulties.

We succeed according to this invention in overcoming these difiicultiesby means of the device illustrated in Fig. 6, where 21 is the outer wallof the tower and 22 the central gas suction pipe. This pipe issurrounded in each compartment by a ring-shaped vessel 44 which is fixedat its top to the pipe 22 in a liquid-tight manner, while at its bottomit is formed with a comparatively narrow annular slit 45 uniformlysurrounding the pipe 22. A feed pipe 46 extending through the outer wall21 of the tower feeds the liquid to the vessel 44, from which it flowsthrough the slit 45 onto a conical deflecting head 47 which may beformed with perforations and projections serving for uniformlydistributing the liquid over the entire surface of the layer of packingmaterial below.

We wish it to be understood that we do not desire to be limited to thedetails of proceeding and construction described in this specificationand illustrated in the drawings, for obvious modifications will occur toa person skilled in the art.

We claim:

1. An apparatus for the manufacture of nitric acid by the absorption ofnitrous gases in a liquid comprising a tower having several packedcompartments arranged in superposition, a bottom inlet for the gases tobe absorbed,

6 a gas suction pipe positioned within said tower and traversing saidsuperposed compartments, the pipe having an open top extending above theuppermost layer of packing and having an outlet for exhaust gases at itsbottom, an inlet for the absorbing liquid, and a bottom outlet for thenitric acid, means for recycling liquid collecting at the bottom of eachcompartment in that same compartment and individual cooling meansadapted to permit individual control of cooling temperatures in therecycling means of each compartment, collar-like vessels surroundingsaid pipe at the top of each compartment except the uppermostcompartment, an annular opening in the bottom of each such vessel inproximity to said pipe, and inlet pipes traversing the wall of saidtower and opening into each vessel.

2. The apparatus of claim 1 including in addition a truncated conicaldeflecting member surrounding said pipe underneath each vessel bottomand extending over a minor portion of said compartment and inlet pipetraversing the wall of the tower and opening into each vessel, wherebyto distribute absorbing liquid throughout said compart ments.

3. The apparatus of claim 1, in which the respective compartmentscomprise substantially horizontal packing supporting partitions, saidbottoms being subdivided into segments, with abutments on the interiorwall of the tower and on the outside of the suction pipe supporting theouter and inner ends of said bottom segments, re-

spectively.

4. The apparatus of claim 1, in which the respective compartmentscomprise substantially horizontal packing supporting partitions withintermediate bottoms underneath said partitions, said bottoms beingdivided into segments, troughs underneath each segment, openings in saidtroughs to afford passage to the gases rising in the tower, said troughsincluding overflow pipes, and an annular collecting pipe connecting theoverflow pipes of any one bottom, whereby to return liquid for recyclingin the same compartment.

References Cited in the file of this patent UNITED STATES PATENTS1,387,170 Poindexter Aug. 9, 1921 2,027,578 De Jahn Jan. 14, 19362,098,953 Christensen Nov. 16, 1937 2,132,663 Voogd Oct. 11, 19382,206,495 Beardsley July 2, 1940 2,384,874 Barr Sept. 18, 1945 FOREIGNPATENTS 315,629 Great Britain July 18, 1929

1.AN APPARATUS FOR THE MANUFACTURE OF NITRIC ACID BY THE ABSORPTION OFNITROUS GASES IN A LIQUID COMPRISING A TOWER HAVING SEVERAL PACKEDCOMPARTMENTS ARRANGED IN SUPEROSITION, A BOTTOM INLET FOR THE GASES TOBE ABSORBED, A GAS SUCTION PIPE POSITIONED WITHIN SAID TOWER ANDTRAVERSING SAID SUPERPOSED COMPARTMENTS, THE PIPE HAVING AN OPEN TOPEXTENDIDNG ABOVE THE UPERMOST LAYER OF PACKING AND HAVING AN OUTLET FOREXHAUST GASES AT ITS BOTTOM, AN INLET FOR THE ABSORBING LIQUID, AND ABOTTOM OUTLET FOR THE NITRIC ACID, MEANS FOR RECYCLING LIQUID COLLECTINGAT THE BOTTOM OF EACH COMPARTMENT IN THAT SAME COMPARTMENT ANDINDIVIDUAL COOLING MEANS ADAPTED TO PERMIT INDIVIDUAL CONTROL OF COOLINGTEMPERATURES IN THE RECYCLING MEANS OF EACH COMPARTMENT, COLLAR-LIKEVESSELS SURROUNDING SAID PIPE AT THE TOP OF EACH COMPARTMENT EXCEPT THEUPERMOST COMPARTMENT, AN ANNULAR OPENING IN THE BOTTOM OF EACH SUCHVESSEL IN PROXIMITY TO SAID PIPE, AND INLET PIPES TRAVERSING THE WALL OFSAID TOWER AND OPENING INTO EACH VESSEL.